The present invention relates to a rod lens array, and image read apparatus and image read system using the rod lens array.
With the recent rapid spread of personal computers, there exists demand for a system for reading an image, such as a photograph and an illustration, using an information processing apparatus for image input purpose, called scanner, processing the read image on a computer, and printing the processed image on a postcard or other document. When using the scanner in a residence, a setting space, utilization convenience, and the appearance of the scanner are important. Accordingly, a small and light-weight scanner is demanded.
As an example achieving down-sizing and down-weighing, there is a scanner using a contact type image sensor, as an image read unit, for reading an original image upon contacting the original image. FIG. 15 is an external perspective view of a conventional contact type image sensor. In FIG. 15, reference numeral 1 denotes a frame for supporting the entire image sensor; and 5, a cover glass for determining a reading surface upon contacting an original image. The frame 1 includes many photodetectors arranged along the length (in the direction of an arrow DM) of the frame 1 as a sensor array. Here, the direction of the arrow DM is denoted as the main scanning direction and the widthwise direction (direction of an arrow DS which is perpendicular to the direction DM) of the frame 1 is denoted as the sub-scanning direction.
FIG. 16 is a cross-sectional view taken along a line B-B' in FIG. 15. A rod lens array 7, as an image forming device, is arranged in a first space 1A, and an illumination device 6 is set in a second space 1B. The first and second spaces 1A and 1B are connected. The illumination device 6 comprises a light guide 61 having the function of guiding the light, emitted from one or more LED light sources 63, in the main scanning direction DM and illuminating an original image 500 by reflecting the guided light on a reflection surface 64 toward the original image 500, and a housing 62, or a frame, having the function for preventing light from leaking from the light guide 61 and fixing the position of the light guide plate 61 so that the original image 500 is effectively illuminated. The light emitted from the above light source 6 illuminates the original image 500 on a cover glass 5, and the rod lens array 7 and a sensor substrate 4, which is arranged in a third space 1C, are arranged so that the reflected light from the original image forms an image on a sensor IC 3 on the sensor substrate 4.
The foregoing image sensor is assembled in such manner that the illumination device 6 is fixed on a clamp face 1D of the frame 1 by adhesive or screws, and the rod lens array 7 is inserted into the first space 1A and fixed on a clamp face 1E of the frame 1 by adhesive or screws. Then, the sensor substrate 4 on which the sensor IC 3 is mounted is fixed on the frame 1 via a frame 2. Thereby, the image sensor is assembled.
The rod lens array 7 is formed in such manner, as shown in FIG. 17, that a plurality of rod lenses 71 are arranged in one or more lines, sandwiched by thin supporting plates, or side plates, 72 and 73, and the spaces between the rod lenses 71 and the supporting plates 72 or 73 are filled with resin to fix the positions of the rod lenses 71.
Each rod lens 71 is made by gradually decreasing index of refraction from the peripheral portion toward the central portion. The diameter of the rod lens 71 is about 0.6 mm, and the thickness of the side plates 72 and 73 is about 0.5 mm. The rod lens array 7 has characteristics of forming an image of the original image 500 on the object plane of the same size on the image plane. Therefore, the rod lens array 7 is suitable for forming an image of an original image on the sensor surface to read the image when the original image is placed on the object plane and photodetection surface is arranged on the image plane.
FIG. 18 shows relationship between the rod lens array 7, an object plane 74, and an image plane 75. The distance between the object plane 74 and the image plane 75 depends upon the characteristics of each rod lens, and is called "conjugate distance" (TC). Further, the distance between the end of the lens and the object plane 74 or the image plane 75 is called "working distance" (L0). Further, the distance denoted by Z0 is the length of each rod lens forming the rod lens array 7, or the height of the rod lens array 7.
As for the rod lens array 7 mainly used in this type of the image sensor, there are two types of lens arrays; those having the conjugate distance TC of 9 mm and those having the conjugate distance TC of 18 mm. When down-sizing is given priority, an image sensor of the former type is used; whereas, when resolution and depth of focus are given priority, an image sensor of the latter type is used. Below, an image sensor using a rod lens array whose conjugate distance TC is 9.1 mm is explained.
FIG. 19 is an image sensor using a rod lens array 7 whose conjugate distance TC is 9.1 mm, and FIG. 20 shows position relationship between the rod lens array 7, the illumination device 6, the cover glass 5, and the sensor substrate 4. Note, the illumination device 6 has a plurality of LED chips arranged in a line on a substrate. The height Z0 of the rod lens array 7 is 4.3 mm, and the distance from the end of the rod lens array 7 to the surface of the sensor IC 3 corresponds to the working distance L0, and it is 2.4 mm. Further, the distance L0' from the end of the rod lens array 7 to the original image 500 is slightly larger than the working distance L0, since there is the cover glass 5 of the thickness d of 1 mm having the index of refraction of about 1.5 between the end of the rod lens array 7 and the original image 500, and the distance L0' is 2.7 mm. Therefore, the distance from the original 500 to the sensor IC 3 is 9.4 mm, which is slightly longer then the conjugate distance TC of the rod lens array 7, and the thickness of the image sensor is about 11 mm including the thickness of the sensor substrate 4.
In order to make the best use of the advantage of the above contact type image sensor to realize a compact image sensor and further minimize the image sensor, utilization of an image forming device having a shorter conjugate distance TC than the above is proposed.
The conjugate distance TC of a rod lens array is known to be determined by the following equations; EQU TC=Z0+2.times.L0L0=-1/(n0.times.A).times.tan(Z0.times..pi./P) P=2.times..pi./A (1)
In the above equations (1), A denotes a distributed constant of the index of refraction of each rod lens, n0 denotes the index of refraction of each rod lens on the optical axis, Z0 denotes the length of each rod lens, P denotes wavelength, and L0 denotes the working distance.
As a method for shortening the conjugate distance TC expressed by the above equations (1), there is a method for slightly increasing the length of the each rod lens forming the rod lens array 7, or the height Z0 of the rod lens array 7. For example, in the rod lens array having the conjugate distance TC of 9.1 mm as shown in FIG. 20, the value of the variables are:
n0=1.639 PA1 A=0.8637 PA1 Z0=4.3 mm
Among the above values, with n0 and A fixed, Z0 is slightly increased to 4.5 mm, and the conjugate distance TC becomes 8.1 mm according to the above equations (1).
FIG. 21 shows a lens array 7 having the height Z0 of 4.5 mm and the conjugate distance TC of 8.1 mm. With this configuration, it is possible to shorten the conjugate distance TC without changing distribution of index of refraction of the rod lenses. In this method, however, the length of the rod lenses, or the height Z0 of the rod lens array is lengthened, and there is a limitation for shortening the conjugate distance TC.
Accordingly, it is impossible to realize the conjugate distance TC of a desirable short length.
Accordingly, as another method for shortening the conjugate distance TC, there is a method of changing the distribution of index of refraction in each rod lens so that the index of refraction from the peripheral portion toward the central portion decreases more rapidly than before. Namely, among the aforesaid constants n0, A and Z0, A is changed.
FIG. 22 is a cross-sectional view of a lens array having different distributed constant. As shown in FIG. 22, since paths of incoming light in each rod lens bend largely within the rod lens, the conjugate distance TC is shortened. In this method, while shortening the conjugate distance TC, the height of the rod lens array Z0 is also shortened. The rod lens array 7 in FIG. 22 has the conjugate distance TC of 4.7 mm, and the height Z0 of 2.1 mm, and the distance L0 between the end of the rod lens array 7 and the surface of the sensor IC 3 is 1.3 mm. The distance L0' between the other end of the rod lens array 7 and the original image is 1.6 mm, which is slightly longer than the distance L0, since there is the cover glass 5 of 1 mm thickness having the index of refraction of 1.5.
An image sensor adopting such rod lenses having a short conjugate distance TC is shown in FIG. 23. As shown in FIG. 23, by fixing the cover glass 5 to the frame 1 while pressing the illumination device 6 inserted in the second space 1B and the rod lens array 7 inserted in the first space 1A against the frame 1 by the cover glass 5, to the frame 1, the light source 6 and the rod lens array 7 are simultaneously fixed. Further, the sensor substrate 4 on which the sensor IC 3 is formed is fixed to the frame 1 by adhesive or screws or by caulking the frame 1. By configuring the image sensor as described above, the thickness of the image sensor is further thinned. Note, in FIG. 23, TC' is a conjugate distance, however, since there is a cover glass 5 instead of atmosphere, the distance TC' is different from the conjugate distance TC expressed by the equations (1). Specifically, TC' is the sum of Z0, L0, and L0'.
FIG. 24 shows relationship between the rod lens array 7 having the conjugate distance TC of 9.1 mm and light emitted from the illumination device 6. The light emitted from the illumination device 6 travels the space between the end of the rod lens array 7 and the original 500 (L0'=2.7 mm) diagonally, and illuminates a focal position of the rod lens array on the original, the read point P. Practically, it is designed to illuminate an area around the point P by .+-.0.2 mm in the sub-scanning direction to cope with tolerance of mechanical configuration. Generally, the illumination device 6 illuminates the original images at an angle between 30 and 50 degrees with respect to the optical axis of the lens array 7.
FIG. 25 shows relationship between the rod lens array 7 having the conjugate distance TC of 4.7 mm, about half the height of the rod lens array shown in FIG. 24, and illuminating light. The light emitted from the illumination device 6 travels the space between the end of the rod lens array 7 and the original image 500 (L0'=1.6 mm) diagonally. Since the working distance L0 between the end of the lens array 7, having a short conjugate length TC, and the focal point is short, it is necessary to place the illumination device 6 near the lens array 7 as much as possible. However, due to the thickness of the housing of the illuminating device 6, it may not be possible to illuminate the point P even though the illumination device 6 is placed at the closest position to the lens array 7. Thus, when an image sensor is configured using a lens array having a short conjugate distance TC, there would be a problem in which an original image may not be read effectively since the illuminating light can not illuminate the point P with enough brightness.
Further, in a case where the rod lens array 7 is placed at some distance from the cover glass 5, it is necessary to provide light shielding member so as to prevent direct light from the illumination device 6, internal reflected light, and stray light from outside of the contact type image sensor from entering the rod lens array 7. However, the shielding member makes it even harder to illuminate the focal point by the light source when a rod lens array of a short conjugate distance is used.